Valencian rice growers usually apply excess N, which results in loss of soil quality because of its negative effects on the soil N2-fixing cyanobacteria (Carreres et al 1996). In Valencia, constraints in the irrigation system have led to lack of control in water management, with difficulties in irrigating after incorporating N fertilizer. The low N efficiency reflects substantial N losses (Fernandez Valiente et al 2000). Improving N efficiency and soil use by preserving and developing the ability of N2-fixing organisms for biofertilization would reduce N fertilizer use and result in ecological, economic, and environmental benefits. N efficiency cannot be improved by split application in Valencia (Carreres et al 1998) but it can be enhanced by slow-release fertilizers (SRF) in Europe (Moletti et al 1989). The relationships between N2 fixation, physical and chemical characteristics of water and sediments, and rates and timing of inorganic N fertilizers were described in previous papers (Quesada et al 1997, Carreres et al 1998). Studies dealing with the effect of SRF on N2 fixation are lacking. This work evaluates the use of different SRF and nitrification inhibitors (NI) in rice fields of Valencia, Spain.